Optimize FixReblogsInFeeds migration (#5538)

We have changed how we store reblogs in the redis for bigint IDs. This process is done by 1) scan all entries in users feed, and 2) re-store reblogs by 3 write commands.

However, this operation is really slow for large instances. e.g. 1hrs on friends.nico (w/ 50k users). So I have tried below tweaks.

* It checked non-reblogs by `entry[0] == entry[1]`, but this condition won't work because `entry[0]` is String while `entry[1]` is Float. Changing `entry[0].to_i == entry[1]` seems work.
  -> about 4-20x faster (feed with less reblogs will be faster)
* Write operations can be batched by pipeline
  -> about 6x faster
* Wrap operation by Lua script and execute by EVALSHA command. This really reduces packets between Ruby and Redis.
  -> about 3x faster

I've taken Lua script way, though doing other optimizations may be enough.

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